codec: rpc tests should listen on ipv4 localhost only.

This ensures that
- net.Dial works just fine on go 1.7 and 1.8
- firewall concerns do not come up, because the server is
  listening for connections outside local machine

Updates #247
- with aim to revert the PR

codec/codec_test.go

@@ -1150,7 +1150,8 @@ func testCodecRpcOne(t *testing.T, rr Rpc, h Handle, doRequest bool, exitSleepMs
 	}
 	srv := rpc.NewServer()
 	srv.Register(testRpcInt)
-	ln, err := net.Listen("tcp", ":0")
+	ln, err := net.Listen("tcp", "127.0.0.1:0") // listen on ipv4 localhost
+	logT(t, "connFn: addr: %v, network: %v, port: %v", ln.Addr(), ln.Addr().Network(), (ln.Addr().(*net.TCPAddr)).Port)
 	// log("listener: %v", ln.Addr())
 	checkErrT(t, err)
 	port = (ln.Addr().(*net.TCPAddr)).Port